CMOS image sensors are beginning to replace conventional CCD sensors for applications requiring image pickup such as digital cameras, cellular phones, PDA (personal digital assistant), personal computers, and the like. Advantageously, CMOS image sensors are fabricated by applying present CMOS fabricating process for semiconductor devices such as photodiodes or the like, at low costs. Furthermore, CMOS image sensors can be operated by a single power supply so that the power consumption for that can be restrained lower than that of CCD sensors, and further, CMOS logic circuits are easily integrated in the sensor chip and therefore the CMOS image sensors can be miniaturized.
Electronic global shutter offers a number of advantages over the commonly used rolling shutter operation for CMOS image sensors. Because the light-induced signal is transferred simultaneously, the artifacts induced by image motion during the readout time of the rolling shutter readout are avoided. Unfortunately, the electronic global shutter operation introduces new challenges to the sensor. One challenge is the inability to do correlated double sampling (CDS). With correlated double sampling, the floating diffusion is reset by turning on the reset gate and this value is sampled and stored. Immediately thereafter, the charge from the photodiode is transferred to the floating diffusion and sampled. The difference between these voltage values is the net signal level from the photodiode, and reduces the noise due to variable reset gate signals and 1/f noise in the source follower in rolling shutter mode. This CDS operation is not possible with the standard 5-transistor electronic global shutter. The signal needs to be stored on the floating diffusion, so even if the reset value is stored for the length of the frame, it will not be correlated in time for each row.
To address the noise issue, 7T global shutter pixels are a promising new technology for low noise, high quality global shutter applications. For example, 7T technology can eliminate the motion artifacts inherent in rolling shutter operation, while also providing correlated double sampling in global shutter operation. The 7T pixel sensor cell works by, for example:                taking an image, allowing the photodiode to convert the photons into electron/hole pairs;        integrating over a shutter time, the collected electrons on the photodiode;        globally transferring the electrons from all the photodiodes simultaneously;        holding charge on the middle hold gate till the row can be read out; and        CDS readout from the hold gate to the floating diffusion.        
However, the disadvantage of the 7T pixel sensor cell is the need for additional chip real estate to accommodate extra transistors, i.e., a total of seven transistors. This, in turn, leads to increased production costs. In addition, pixel sensor cells (e.g., CMOS imagers) can suffer added noise when the transfer of charge from the photodiode through the transfer gate is incomplete. For 7T transistors, this challenge is more significant as there are now additional transfers that must be made with near perfect efficiency. Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.